1. Field of the Invention
The present invention relates to an antenna apparatus used as antenna mounted on a surface of a vehicle or used as a built-in antenna for a portable telephone or the like.
2. Description of the Related Art
The antenna of a portable telephone suffers a changeable frequency characteristic depending on the proximity of the user's body or the like. To mitigate the change, the antenna of a portable telephone must be broadband.
An antenna shown in FIG. 1 is a conventional antenna. The antenna is a built-in antenna which is set on one surface, i.e., ground plane 100 of a square internal housing 101 made of a ground conductor (ground plane) inside an external housing made of an insulator such as a plastic in a wireless communication device. This antenna is constituted by a planar inverted-F antenna made up of a first and second planar antenna elements 104 and 105, and a third planar antenna element 106 interposed between the ground plane 100 and the second planar antenna element 105. The second planar antenna element 105 is connected to a feed line 103 at a node 111, whereas the third planar antenna element 106 is connected to the feed line 103 at a node 112.
A radio circuit 113 is connected to the feed point 102 and transmits and receives a radio wave via the first, second, and third planar antenna elements 104, 105, and 106.
The antenna shown in FIG. 1 serves as a broadband antenna by adding the third planar antenna element 106 to the planar inverted-F antenna. This antenna, which occupies a wide area in mounting and is difficult to design, was reported by the present inventor (No. 675) in the 1986 IEICE National General Conference in Japan.
In recent years, terminals such as for wireless communication devices are being downsized for progressing its portability. Demands have arisen for a small structure in which an antenna as shown in FIG. 1 is mounted on a circuit board and parts are mounted immediately below a planar antenna element. However, the antenna shown in FIG. 1 has two, third and second, planar antenna elements, which poses limitations on downsizing of parts mounted on the circuit board 100.
The antenna shown in FIG. 1 requires a long time for design. This antenna comprises the first, second, and third planar antenna elements 104, 105, and 106. The widths and heights of the first, second, and third planar antenna elements 104, 105, and 106, and their area which is the product of the widths and heights are included in parameters which determine the frequency characteristic of the antenna. Correlation parameters between the first, second, and third planar antenna elements 104, 105, and 106 cannot be ignored. A model to be input to an electromagnetic simulation is difficult to formulate. For an experimental approach, many parameters must be taken into consideration. It takes a long time to optimize the dimension values of the structure. Since the design guideline values of the antenna have not been determined, desired broadband characteristics are very difficult to obtain. As described above, in a conventional broadband planar inverted-F antenna as shown in FIG. 1, an unnecessary mounting area and design difficulty are left unsolved.